Close linkage was found to exist between Akv-2, an ecotropic provirus found uniquely on chromosome 16 of AKR/N mice, and the immunoglobulin Lambda light chain locus, Igl-1. No recombinants between the Igl-1 locus and AKv-2 were found by Southern blot analysis of DNA obtained from progency of the backcross of (AKR/N x SJL/J)F1 to SJL/J, indicating that these genes map within 5.9 cM of each other. A probe specific for the flanking sequence of AKv-2 was used to detect the provirus, while one specific for the Igl-1 constant region was used to determine which allele of the structural gene was expressed in the backcross mice. The constant region of Igl-1 differs between AKR/N and SJL/J with respect to a site for the restriction endonuclease KpnI.